Impregnated fibrous board and method



United} States Patent IMPREGNATED FIBROUS BOARD AND METHOD OF MAKINGSAME Walter P. Ericks, Lockport, N. Y., assignor to The Upson Coplipany,Lockport, N. Y., a corporation of New Yo No Drawing. Original No.2,629,648, dated February 24, 1953, Serial No. 103,528, July 7, 1949.Application for reissue June 8, 1954, Serial No. 435,390

19 Claims. (Cl. 117-140) Matter enclosed in heavy brackets [fl appearsin the original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

This invention relates to dimensionally stabilized materials ofcellulose fibers, particularly cellulose structural materials, and tomethods for stabilizing such materials against dimensional change causedby change in the humidity of the environment surrounding such cellulosematerials. More particularly, the invention relates to the stabilizationof structural cellulose fiber boards as well as wood and paper andfabrics made of cotton, linen and other cellulose materials to renderthem more resistant to dimensional changes resulting from variations inthe ambient humidity and to improve the strength of such products.

It is well known that materials made up entirely or predominantly ofcellulose fibers expand and contract with variations in humidity in theambient atmosphere, such materials suffering an increase in theirdimension upon absorption of moisture from the atmosphere and acontraction when moisture is given up to the atmosphere upon a decreasein thehumidity thereof. It is also well known that in articles whereinfibers are directionally oriented, such expansion and contractionusually occurs to the greatest extent in a direction perpendicular tothe predominant direction of the fibers. The present invention is,therefore, adapted particularly in preventing or minimizing thedimensional change which occurs across the fibers with change inhumidity in cellulose materials, although it also reduces dimensionalchange in the direction of the fibers with humidity change.

Various expedients have been heretofore employed for the purpose ofdimensionally stabilizing materials made up predominantly of cellulosefibers as, for instance, plywood, wood boards, pulp products andcombinations thereof, and solid paper boards. A degree of dimensionalstabilization is obtained in the manufacture of plied or laminatedarticles by arranging the laminations with their fiber directionsdisposed angularly to one another rather than parallel. stabilization isobtained, the operation is laborious since it requires cutting andproper selection and assemblage of the plies.

It has also been suggested to densify the products under heavy pressureand to thereby set the cellulose fibers.

Very expensive presses and extensive auxiliary equipment is required forthis operation, and the product lacks low density and some of theflexibility desired for many uses of structural cellulose fiber board.

In my copending application Serial No. 627,966, filed November 10, 1945,and now abandoned, of which this is a continuation-in-part, I havedisclosed that certain organic compounds having at least two hydroxylgroups in their molecules, particularly partial esters of polycarboxylicacids and polyhydric alcohols having at least one hydroxyl group in theresidue derived from the polyhydric alcohol and at least one carboxylgroup in the residue Although improvement in dimensional derived fromthe carboxylic acid, stabilize structures made up predominantly ofcellulose fibers against expansion and contraction due to variation inatmospheric humidity. I have also disclosed in my copending application.Serial No. 103,526 filed July 7, 1949, that other compounds having bothhydroxyl and carboxyl groups, such as bydroxy carboxylic acids, are alsoeffective stabilizers against such expansion and contraction. Also in mycopcnding application Serial No. 103,527, filed July 7, 1949, I havedisclosed that polyhydric alcohols have asimilar stabilizing effect.

In accordance with the present invention, I have found that materialsmade up predominantly of cellulose fibers may be wholly or partiallystabilized against dimensional change by introducing into such cellulosematerials certain specified chemical compounds which also appear to havea particular afiinity for the cellulose fibers. Compounds which producedimensional stabilization are orgarlic in nature and have at least twohydroxyl groups, at least one of which is a part of a carboxyl group,and possess certain other characteristics with respect to volatility.The series of compounds possessing dimensional stabilizingcharacteristics are the polycarboxylic acids.

In this case, as well as in that dealing with the partial esters and thehydroxy carboxylic acids mentioned above, at least one of the hydroxylgroups forms a part of a carboxyl group. The polycarboxylic acids areusually either soluble in water or in low molecular weight aliphaticalcohols or ketones or mixtures of these solvents. When in solventsolution, they rapidly penetrate into the fibrous structures, betweenthe fibers and into the fiber cells, and in fact many of them rapidlypenetrate such fibrous structures even in the absence of a solvent. Inmost cases, however, greater dimensional stability is obtained when thepolycarboxylic acids are employed in solution in a solvent forimpregnating the fibrous structures. Further properties andcharacteristics of the stabilizing chemicals will be more fullydescribed hereinafter.

For purposes of illustration only, the invention will be described indetail in its application to the production of dimensional stability inlaminated structural cellulose fiber boards. Such products are bestexemplified upon the market by the structural building panels sold underthe name Upson Board]; These cellulose fiber boards are generallymanufactured from so-called fiber boards, that is, a fiber sheet with acaliper greater than about 0.030 inch. These fiber boards are assembledand bonded to one another to produce a laminated or plied articlehaving, for instance, from two to about seven plies. The resultinglaminated structural cellulose fiber board occurs in standard sizedpanels of from /s inch to inch or more in thickness, and of specifiedlengths and widths. The original cellulose board is manufactured fromany conventional type of cellulose pulp stock as, for instance, groundwood.fiber, chemical wood fiber, rag fiber and other conventional pulpfibers and mixtures thereof. The initial cellulose board whichconstitutes the individual ply may be made either upon a conventionalcylinder machine, as is generally the case, or may be made upon aFourdrinier machine. It will be understood, however, that the inventionis of general application to structural cellulose materials as, forinstance, fiber insulation board, sound absorbing board, table topboard, structural board for the interior of an airplane, and the like.

The compounds employed to effect a stabilization in the structuralcellulose fiber board against dimensional change induced by change inhumidity may be introduced into the fibers from which the board is made,into the individual plies of the ultimate laminated structure or intothe final laminated assembly itself. The choice of the place ofintroduction of the stabilizing compound and the manner in which it isto be introduced will be dictated by the type of fiber available and thetype of structural panel to be produced.

Thus, when operating a closed board machine system wherein all water isrecycled, the impregnating compound may be added to the beater or thestock prior to paper formation, as for instance in the head chest,assuming that a stabilizing compound has been chosen which is notreadily subject to hydrolytic change at the temperature and pH of thepulp suspension. Or, the impregnating compound may be added at any otherpoint in the wet end of the machine.

Where the individual cellulose structural board is already formed, thestabilizing compounds may be introduced into the board by immersing theboard in the compound or a solution thereof or by impregnating the boardwith a spray containing the treating compound or by applying it withpatting rolls, all conventional methods of impregnation. Where alaminated board has already been formed by bonding a plurality ofindividual boards together, the resultant laminated article may beimmersed in the stabilizing compounds or a solution thereof and theimpregnated board subsequently dried. The impregnation under suchcircumstances will generally be desirably performed by subjecting theboard to vacuum, at which time occluded gases and volatile materials areremoved from the board, then permitting the impregnating solution toflow into the evacuated chamber containing the board generally placedtherein in an upright position and spaced apart, whereby the boards areenveloped in the solution which is, in effect, forced into the boards.This penetration of the solution may then be increased by relieving thevacuum and, if desired, raising the pressure above that of theatmosphere to enhance the speed and depth of penetration.

It is therefore an object of the present invention to provide a simpleand inexpensive impregnating method for dimensionally stabilizing andstrengthening structures made up substantially of cellulose and toproduce dimensionally stabilized cellulose products.

In broad aspect, therefore, the invention comprehends the incorporationinto structural cellulose fibrous materials of' a polycarboxylic acid ormixture of such acids whereby the usual expansion and contraction ofsuch cellulose materials is considerably minimized by reason of changein humidity conditions in the atmosphere surrounding such materials.This stabilizing effect is dependent upon the quantity of thestabilizing compound incorporated in the fibrous cellulose structuralelement. Effective dimensional stabilization has been accomplished byincorporating in the fibrous material from about 2 to 50% of thestabilizing compound based upon the weight of oven dried fiber. Theexact quantity to be incorporated into the cellulose material will bedictated by the type of material, the type of polycarboxylic acidemployed as a stabilizing compound, and the amount of the usualexpansion or contraction which it is desired to remove. Thus, undercertain conditions of use, the removal of so little as 12 or 15% of thenormal expansion or contraction of a cellulose structural material maybe suitable, while in other conditions of use, it may be desired toremove 50, 60 or 70% or more of the normal expansion or contractionencountered with a particular change in humidity conditions in thesurrounding atmosphere.

The stabilizing compound may be incorporated into the cellulose fibers,whether the same be in fibrous form, unfibrated or felted fibrous form,by the employment of aqueous solutions, solutions in hydrophilicsolvents, or mixtures thereof with water or in some instances may beincorporated without the employment of a solvent. However, the usualmode of incorporation will be to employ as an impregnating solution anaqueous or hydrophilic solution of the stabilizing compound.

The cellulose structural materials may be treated with the impregnatedstabilizing material or solution thereof at substantially any desiredtemperature, although the 4 usual impregnating temperatures willrangebetween 20 C. and 50 C. However, temperatures as high as C. mayfrequently be employed.

While the actual mechanism of the stabilizing action of the presentinvention for cellulose fibers is not fully understood, it is believedthat their penetrating power and their fixation on and in the cellulosefibers is due to the particular molecular structure, that is to say, thepresence of hydroxyl groups in both the cellulose and the stabilizingmaterial. I

After their incorporation in the cellulose material to be dimensionallystabilized, the stabilizing compounds show considerable resistance toremoval by water and solvents, and it is believed, therefore, thatprobably there is some loose form of physico-chemical combinationbetween the cellulose molecule and the stabilizing chemical. Thisresistance to removal of the stabilizer by water and solvents is quitemarked, particularly if the impregnated cellulose products are heated toelevated temperatures as, for instance, between 100 C. and 200 C. It isfurther believed that the fixation of the stabilizing materials in andon the cellulose fiber may be due to the ability of the molecules of thestabilizing materials to react with each other, as well as with thecellulose, whereby polymerization takes place with the formation of longchain molecules of high molecular weight. The presence of carboxylgroups in the polycarboxylic acid suggests that, on heating, thehydroxyl groups of the polycarboxyl acid reacts with the hydroxyl groupsof the cellulose to modify the chemical structure of the latter. It isbelieved that such modification of the cellulose results in increaseddimensional stability of the cellulose structural material and alsoincreases its strength and water resistance.

The fixation of the stabilizing compounds in and on the cellulose fiberscan be enhanced by employing them in combination with thermosettingresins, which in their partially reacted state are soluble in thevolatile, hydrophilic solvents for the stabilizers such as water, lowmolecular weight alcohols and ketones or mixtures thereof. Thethermosetting resins, after setting, are believed to cover and protectthe stabilizing materials in and on the cellulose fibers from attack bysolvents. In this connection, it is further believed that thestabilizing materials penetrate farther into the cellulose fibers thanthe thermosetting resins, thus producing a protective coating ofthermosetting resins.

The incorporation of thermosetting resins into the cellulose structuremodifies to some extent the etfect of the polycarboxylic acid in such away that the hardness and water resistance of the resulting cellulosefiber structures impregnated by the stabilizers are increased. Therequirement of the properties determined by the ultimate use of theresulting article will guide the selection of the stabilizing materialand its use either separately or jointly with a thermosetting resin.

Suitable thermosetting resins which may be employed in combination withthe stabilizing material of the present invention include phenolformaldehyde, urea formaldehyde, and melamine formaldehyde, which aresoluble in the volatile, hydrophilic solvents employed. Any otherthermosetting resins which in their partially reacted state have theproperty of being soluble in such solvents may also be employed. Theresins become insoluble and infusible upon advancement and preventattack by water or solvents upon the stabilizing materials and reactionproducts thereof deposited in and on the cellulose fibers. The amount ofthermosetting resin may be varied within a considerable range, forexample, between 5% and 50% of thermosetting resin in the finalcellulose fiber product based on the dry weight'of fiber.

The advantages gained in impregnating articles made up of cellulosefibers with polycarboxylic acid alone and in combination withthermosetting resins are shown by the following examples which are to betaken in an illustrative rather than a limiting sense. In securing thedata for such examples the procedure outlined below was followed.

A board prepared on a cylinder paper" machine from used news fiber wascut into strips measuring 0.051" x 2" x 12", extending in its largestdimension perpendicular to the predominating direction of fibers in theboard. The strips were immersed into the impregnating solution kept at50 C. and they were allowed to remain therein until the board was wettedto its center.

The time required for complete impregnation was within a range of 1 to 8minutes, and on the average was about 4 minutes. g

The dry strip was weighed before impregnation and, after the immersionin the impregnant, dried by heating at 125 C. for 30 minutes. weight,the quantity of active ingredients deposited within the board, in and onthe fibers, was determined. The strips were accurately measured dry andthen conditioned in a humidifying chamber, kept at 90% relative humidityand 38 C. for about 48 hours, at the end of which period the strips hadabsorbed a maximum of moisture and usually showed no further increase inexpansion. The total expansion of each of the unimpregnated controlstrip and of the impregnated and dimensionally stabilized strip was thusdetermined. The difference in the amount of expansion between thecontrol strip and the stabilized strip represents the amount of normalexpansion removed by means of the dimensional stabilization treatment;the difference in expansion between the two strips divided by the totalexpansion of the control strip times 100 is the percent of normalexpansion removed.

The flexural strength of the strip was determined by the known method ofapplying a load required to break the strip.

EXAMPLE 1 Cellulose fiber boards were immersed for 4 minutes insolutions kept at 50 C. and containing 25%, 12.5%, 6.25% and 3.125%adipic acid dissolved in isopropanol. The impregnated boards were driedand heated for 20 minutes at 125 C. The resulting boards contained 30%,16%, 12% and 8% of adipic acid, respectively, and showed a reduction inexpansion and contraction in varying humidity of the atmosphere by 94%,64%, 52% and 47%, respectively.

Cellulose fiber boards impregnated with isopropanol solutions containing25%, 12.5%, 6.25% and 3.125% of a solute impregnant composed of equalquantities of adipic acid and liquid cresol-formaldehyde resin capableof advancement contained, after drying and heating, 24%, 11%, 7% and4.1% of the above dimensional stabilizing impregnant and showed 84%,56%, 42%

and 34% of reduction in their contraction and expansion,

respectively.

Impregnated and subsequently dried and heated boards containing 29%,16%, 11% and 3.2% of stabilizing ingredients composed of equalquantities of adipic acid and urea-formaldehyde resin possessing thecharacteristic of advancement to an infusible, insoluble stage onheating, lost 81%, 49%, 24% and 17% of their contraction and expansion,respectively. 1

All impregnated boards possessed water-resistance, rigidity and strengthsuperior to those of unimpregnated boards.

EXAMPLE 2 Cellulose fiber boards impregnated with sebacic acid solutionsof various concentrations in isopropanol contained, after drying andheating, 4.2%, 9%, 16% and 27% of sebacic acid and showed losses of 42%,61%, 91% and 95%, respectively, in their contraction and expansion invarying humidity of the atmosphere.

Impregnated and subsequently dried and heated boards containing 2.2%,3.2%, 12% and 21% of active ingredienst composed of equal quantities ofsebacic acid From the difference in and liquid cresol-formaldehyde resinlost 35%, 47%, 61% and 82% of their contraction and expansion,respectively.

Impregnated and subsequently dried and heated boards containing 3.1%,8.7%, 20% and 27% of active ingredients composed of equal quantities ofsebacic acid and liquid urea-liormaldehyde resin lost 27%, 61%, 85% and93% of their contraction and expansion, respectively, in varyinghumidities of atmosphere.

EXAMPLE 3 Cellulose fiber boards were impregnated with solutionscontaining varying quantities of mixed aromatic polycarboxylic acidsdissolved in a solvent composed of 20 parts of isopropanol and 80 partsof water. In accordance with the information of the supplier, the mixedaromatic acids were prepared by controlled oxidation of bituminous coaland contained three or more carboxyl groups per molecule of benzene.After evaporation of solvents and heating of the board for 15 minutes atC. some of the boards lost more than 50% of their contraction andexpansion. A substantial improvement in di mensional stability was alsoobtained when the dimensional stabilizing impregnant was composed ofequal parts of mixed aromatic acid and urea-formaldehyde orcresol-formaldehyde resins. All impregnated and subsequently heatedboards showed improvement in waterresistance and fiexural strength.

EXAMPLE 4 Cellulose fiber boards were impregnated with solutionsmaintained at 50 C. and containing various concentrations of malonicacid alone and its combinations with either cresol-formaldehyde orurea-formaldehyde resin. After evaporation of solvents and heating allof the impregnated boards showed an improvement in dimensionalstability, water-resistance and fiexuralstrength.

EXAMPLE 5 Impregnating solutions were prepared by dissolving 50 parts ofmaleic anhydride in parts of water kept at 50 C. Cellulose fiber boardswere immersed into the solution and kept beneath its surface for 4minutes, at the stabilizing impregnant composed of equal parts of maleicanhydride and cresol-formaldehyde resin dissolved in a solvent composedof equal parts of isopropanol and water. The impregnated boards, aftertheir drying and heating at 125 C. for 30 minutes, contained 4.2%, 12%and 14% of active ingredients and lost 14%, 36% and 43% of theiroriginal property of contracting and expanding in varying humidity ofthe atmosphere.

An improvement in dimensional stabilization was also obtained when theboards were impregnated with a solution prepared by dissolving maleicanhydrides and ureaformaldehyde resin in 20% aqueous isopropanolsolution followed by heating of the impregnated boards at 125 C. for 30minutes. All impregnated boards showed improvement in water-resistanceand fiexural strength.

A still further improvement in dimensional stability of impregnatedboards was obtained when they were subjected to moderate pressure duringtheir heat treatment, on the order of 200 lbs. per square inch.

7 EXAMPLE 6 Impregnating solutions containing 3.125%, 6.25% and 12.5% offumaric acid were prepared by dissolving required quantities of the acidin isopropanol. Cellulose fiber boards impregnated with' the abovesolutions contained 3.2%, 7.2% and 19.8% of fumaric acid, respectively,and showed losses in their original property of expanding andcontracting in varying humidity of the atmosphere by 27%, 31% and 51%,respectively.

impregnated and subsequently heated boards containing 2.1%, 4.2% and12.5 of active ingredients composed of equal quantities ofcresol-formaldehyde resin and fumaric acid lost, after their drying andheating, 30%, 36%, and 44% of their original property of contracting andexpanding due to varying humidity conditions of the atmosphere.

Impregnated and subsequently heated boards containing variouspercentages of fumaric acid and urea-formaldehyde resin showed alsoimproved dimensional stability.

All impregnated and heat treated boards possessed improvedwater-resistance and fiexural strength.

EXAMPLE 7 Cellulose fiber boards impregnated with isopropanol solutionsof succinic acid of various concentrations and containing, afterheating, 5.2%, 9% and 17% succinic acid, lost 34%, 44% and 70%,respectively of their original property of contracting and expandingwith variation of humidity in the surrounding atmosphere.

Cellulose fiber boards possessing improved dimensional stability werealso obtained when they were impregnated with solutions containingsuccinic acid and urea-formaldehyde or cresol-formaldehyde resin andsubjected to heat treatment thereafter. The boards thus treated alsopossess improved rigidity, water-resistance and flexural strength.

EXAMPLE 8 Impregnating solutions were prepared containing variousconcentrations of a solute composed of phthalic anhydride alone or inconjunction with one of the thermosetting resins, such asurea-formaldehyde or cresol-form" aldehyde, and a solvent composed of 1part of isopropanol and 4 parts of water. Cellulose fiber boardsimpregnated with these solutions possessed, after heating, improveddimensional stability, rigidity, water-resistance and fiexural strength.

EXAMPLE 9 lmpregnating solutions were prepared by dissolving inisopropanol required quantities of carbic anhydride to contain 3.125%,6.25% and 12.5% of the latter.

Cellulose fiber boards impregnated with above solutions maintained at 50C. and subsequently heat treated, contained 4.2%, 7.2% and 10% carbicanhydride and showed a reduction in contraction and expansion by 19% 26%and 35%.

Cellulose fiber boards impregnated with solutions of variousconcentrations containing equal quantities of carbic anhydrides andcresol-formaldehyde resin dis solved in isopropanol contained 3.2%, 6.3%and 10% of above active ingredients and lost, after drying and heatingfor 30 minutes at 125 C., 22%, 28% and of their original property ofcontracting and expanding in varying humidity of the atmosphere.

Where impregnation of the fibers is attempted prior to the preparationof a fiber board. economic and operational restrictions will narrow theselection of the polycarboxylic acids employed under such circumstancesto those which are soluble in water. Comminuted cellulose fibers can beimpregnated, however, with the stabilizing chemicals dissolved inorganic solvents and structural members made therefrom show excellentdimensional stability under extremes of humidity conditions. This isshown in the following example:

8 EXAMPLE A An aqueous pulp suspension of a consistency of 1% wasprepared containing 18% concentration of adipic acid based on solution.Sheets of fiber board were prepared from this pulp,cut to size and theexpansion determined by increasing the humidity from 0% to .When thisexpansion was compared with that of board made from another portion ofthe same pulp without the presence of the stabilizer, it was found thata 13% content of the acid in the board, based on the weight of dryfiber, eliminated 45% of the normal expansion.

The same type of results were obtained when applying a solution of thestabilizing chemicals to the wet end of the paper making machine. Thisoperation gives somewhat greater flexibility in the choice ofstabilizing compound to be employed, as compared with addition to thebeater or head chest, for example, since it is entirely practicable touse organic solvent solutions of the stabilizer, for instance, asolution made of equal parts water and isopropyl alcohol and containing2% concentration of sebacic acid. When applying such a solution to thewet lap in amounts to provide 2.5% of acid in the board on a dry fiberbasis, reductions in the normal expansion of 26% were obtained. At theselower dilutions, good results were obtained but, in many instances,operating technique will dictate the employment of relativelyconcentrated solutions when application is made to the wet lap.

Laminated cellulose structural fiber board may be impregnated with thedimensional stabilizer in any suitable fashion although immersion in thedimensional stabilizer or a solution thereof is recommended. In general,the temperature of the liquid in which the laminated cellulosestructural fiber board is immersed will be at room temperature. Where alaminated product of an exceptionally high caliper is to be impregnated,the temperature of the liquid may be elevated to facilitate penetration.The laminated board may be soaked in the impregnating solution untilsuch time as the desired quantity of dimensional stabilizer has beenabsorbed by or combined in some [physico-chemical] manner with thecellulose.

It may be found expedient when treating laminated cellulose structuralfiber boards, or other cellulose elements which are relatively rigid, topack the same in a chamber preferably in an upright position, having theboards spaced slightly apart to facilitate free circulation. It willalso be found expedient to subject the chamber to vacuum whereby gasesand other volatile materials, which interfere with free penetration ofthe solution into the board, are removed. Liquid containing the dimensional stabilizer is then admitted to the evacuated chamber containingthe cellulose material and penetration throughout the body of thecellulose elements is facilitated. The impregnated boards are thenremoved from the solution and passed through any conventional form ofdrier.

Examples of polycarboxylic acids which may be employed in the presentinvention are oxalic, malonic, succinic, methyl-malonic, glutaric,propane-alpha-beta-dicarboxylic, ethylmalonic,butane-alpha-dcltwdicarboxylic, adipic, beta methyl propane alpha-betadicarboxylic, mixed polycarboxylic aromatic acids obtained by thecontrolled oxidation of bituminous coal, pimelic, diethylmalonic,suberic, azelaic, sebacic, decamethylene-dicarboxylic, tridecandioic,tetradecandioic, octadecandioic, nonadecandioic, fumaric, maleic,glutaconic, mesaconic, citraconic, mueonic, alpha-carboxyglutaric,beta-carboxy-pimelic, camphoronic, aconitic, cyclopentendioic,1,1-hexahydrophthalic, camphoric, phthalic, Z-carboxyphenylacetic,phenylsuccinic, benzalmalonic, naphthalenedicarboxylic-l,8;diphenyl-carboxylic-2,2'; cyclopropanetetracarboxylic l,l,2,2;naphthalene tetracarboxylic l,4,5,8; dimer-abietic, dimer-linoleic,dimer-linolenic ant the like and their anhydrides, such as phthalic,maleic,

(3,G-endomethylene-tetrahydrophysical characteristics, for employment asstabilizers in.

accordance with the present invention. One of the important physicalrequirements of the stabilizer is that it be soluble in water orvolatile water-miscible solvents such as lower aliphatic mono alcoholsor ketones, or mixtures thereof. Also, in general, the greater itssolubility in one of such volatile solvents, the greater its penetrationinto the cellulose fibers and the greater its stabilizing effect. Thelatter is true regardless of whether the stabilizing material isemployed in solvent solution during impregnation.

Another' important physical characteristic of the stabilizer is that itbe substantially non-volatile under all temperature conditions likely tobe encountered. That is to say, it should have a boiling point at leastas high as 150 C. and preferably substantially higher at atmosphericpressure.

As to the solvents which may be employed for making up an impregnatingsolution, water is the preferred solvent and will ordinarily be employedalone if the stabilizing material is soluble therein in all portions. Ifnecessary to obtain solution of the stabilizing material,

volatile water-miscible organic solvents such as monohydroxyl aliphaticalcohols, containing three carbons or less or aliphatic ketonescontaining five carbons or less may be employed either alone or inadmixture with each other or with water. By way of example, methyl,ethyl and propyl alcohols are particularly suitable and dimethyl,diethyl, methyl ethyl, methyl propyl or ethyl propyl ketones are alsosuitable. Such solvents or solvent mixtures should have a boiling pointsubstantially below that of the stabilizing mixture, i. e., a boilingpoint not above approximately 105 C. at atmospheric pressure. Suchsolvents may be termed volatile hydrophilic solvents and for purposes ofthis application the term volatile hydrophilic solvent is defined aswater, a watermiscible organic solvent or mixtures thereof having aboiling point not greater than 105 C.

In lieu of the polycarboxylic acid as the dimensional stabilizer, theesters of such acids with low molecular weight alcohols may be employed.It is believed that such esters, either by hydrolysis or alcoholysiswith the cellulose, become effective for the physico-chemicalcombination believed to constitute the mechanism of the stabilizingaction.

To summarize, the stabilizing materials in accordance with the presentinvention are either aromatic or aliphatic polycarboxylic acids ormixtures of such acids. In addition to such chemical requirement, thestabilizing materials should be soluble in all proportions with at leastone of the volatile hydrophilic solvents as defined above and shouldhave a boiling point at least as high as 150 C. Since solubility involatile hydrophilic solvents depends upon several factors such as thenumber of hydrophilic groups, for example, carboxylic groups, thesaturation of the compound and arrangement of carbons as well as thenumber of carbons, it is impossible to more definitely specify thenature of the effective compounds by chemical characteristics. The samesituation exists as to the boiling point of the eflective compounds.

The concentration of the impregnating solution can vary from 100%polycarboxylic acid in the case of lowviscositypolycarboxylic acids downto a concentration as low as 1%. In general, however, the best resultsare obtained when polycarboxylic acids are employed in a solution havinga concentration ranging between approximately 5% and 60%. Also, ingeneral, the dimensional stability obtained does not decreaseproportionately with the decrease in concentration of the impregnatingsolution. When very dilute solutions are employed, a disproportionatelyhigh degree of stabilization is obtained. Thus the concentration of thepolycarboxylic acid in the impregnating solution may range fromapproximately 1% to and the amount of polycarboxylic acid retained inthe fibrous material may range from approximately 1% to 50%, thepreferred range being between approximately 3% and 50%.

The temperature of the impregnating solution during impregnation has aneffect upon the results obtained. That is to say, the rate ofpenetration of the polycarboxylic acid into the fiber structure, thequantity of deposition of the acid in the fiber structure and theresulting improvement in dimensional stability not only de pends uponthe concentration of the polycarboxylic acid in the solution but alsodepends on the temperature of the impregnating solution. The higher thetemperature at which impregnation is carried on, the greater the amountof polycarboxylic acid retained in the fibrous structure for a givenconcentration of the impregnating solution, and the more effective isthe dimensional stabilization. The usual impregnating temperature willrange between 20 and 60 C., although any temperature below that at whichrapid vaporization of the impregnating solution takes place may beemployed. Such temperature will not ordinarily be above 100 C.

The polycarboxylic acids deposited in the fibrous products exhibitconsiderable resistance to removal by water and solvents, particularlyif the impregnated fibrous products are heated during drying orthereafter to temperatures between approximately 100 and 200 C. Thisindicates the possibility of a reaction between the cellulose and thepolycarboxylic acids to modify the cellulose. Such modification of thecellulose is beneficially reflected by the increased dimensionalstability and the increased strength and decreased water-absorptivity ofthe resulting product.

The affinity of the stabilizing materials of the present invention forcellulose fibers, their penetration powers and the reason for theirfixation on and in the cellulose fibers is not fully understood. From aconsideration of their molecular structure, it may be supposed thattheir affinity for cellulose fibers depends upon the presence ofcarboxylic groups in the polycarboxylic acid and the presence ofhydroxyl groups in the cellulose. Their power of penetration is possiblydue to the presence of balanced hydrophilic and hydrophobic groups inthe molecules which is characteristic of surface active materials. Thesetwo properties apparently facilitate the deposition of the stabilizingmaterials in and on the cellulose fibers.

In processing the impregnated cellulose fiber materials, heating to atemperature which produces further reaction of the stabilizing materialboth with itself and with the cellulose is desirable in order to obtainthe maximum effect of the present invention. Such temperatures willusually lie between 100 and 200 C. but in any case, the uppertemperature should not exceed that at which scorching or volatilizationof the stabilizing material may occur during the period of heating. Thepermissible upper temperature limit varies with the duration of heatingand also with the type and quantity of impregnating substances in thecellulose material. The time of heating also varies with the temperatureand the type and amount of stabilizing material or mixture withthermosetting resin retained in the cellulose fiber sheets but willusually range between 3 and 60 minutes. Such heating is ordinarilyrequired to obtain the protective effects of the thermosetting resinswhen such resins are employed as part of the impregnant but substantialdimensional stabilization approaching that obtained after a heattreatment usually results from merely drying the impregnated structuresat low temperatures, regardless of whether thermosetting resins arepresent.

My invention provides an impregnating solution and method forimpregnating cellulose fiber materials which increases the rate anddepth of penetration of the impregnating solution, improves the fiexuralstrength of the cellulose fiber material and stabilizes it againstcontrac- 11 tion and expansion due to varying atmospheric humidity whileat the same time reducing its absorption of water. Being acidic innature, due to the carboxyl groups therein, the polycarboxylic acids ofmy invention are particularly adapted for use in combination withthermosetting resins,

increasing the curing rate of the resins, eliminating the use of mineralacid catalysts for the resins and their detrimental effects, retardingthe gelling of the resins and prolonging their useful life, while theresins, on the other hand,- improve the fixation and water-resistance ofthe stabilizing compositions and reaction products thereof deposited inand on the cellulose fibers. In many instances, the polycarboxylic acidsserve as powerful solvents for the thermosetting resins, therebyreducing the viscosity of the impregnating solutions, and beneficiallyaffecting the impregnation of the cellulose fiber materials. Throughsuch advantageous characteristics, my invention provides a method forimpregnating and improving materials made up of cellulose fibers withoutthe necessity of resorting to the time-consuming, laborious andexpensive steps of arranging laminations with crossing fibers, orapplying heavy pressures which density, crush and injure the fiberstructure. I

What is claimed is:-

l'. The method of stabilizing fiber [wall] boards consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnantconsisting essentially of between approximately 1% and 50% of at leastone polycarboxylic acid, said polycarboxylic acid being in solution in avolatile hydrophilic solvent, and drying the resulting impregnatedboards, said polycarboxylic acid being soluble in said volatilehydrophilic solvent and having a boiling point at least as high as 150C.

2. The method of stabilizing fiber [wall] boards consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a solution in a volatilehydrophilic solvent of at least one polycarboxylic acid and drying theresulting impregnated boards, said polycarboxylic acid being soluble insaid volatile hydrophilic solvent and having a boiling point at least ashigh as 150 C., and said solution having a concentration of saidpolycarboxylic acid between approximately 1% and 60%.

3. The method of stabilizing fiber [Wall] boards consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a solution in a volatilehydrophilic solvent of adipic acid and drying the resulting impregnatedboards, said solution having a concentration of said adipic acid betweenapproximately 1% and 60%.

4. The method of stabilizing fiber [wall] boards consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a solution in a volatilehydrophilic solvent of sebacic acid and drying the resulting impregnatedboards, said solution having a concentration of said sebacic acidbetween approximately l% and 60%.

5. The method of stabilizing fiber [wall] boards consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a poly -12 carboxylic aromaticacid in solution in a volatile hydrophilic solvent, and drying theresulting impregnated boards.

6. The method of stabilizing fiber [wall] boards consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a polycarboxylic aliphatic acid insolution in a volatile hydrophilic solvent, and drying the resultingimpregnated boards.

7. The method of stabilizing fiber [wall] boards con sisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a solution in a volatilehydrophilic solvent of a mixture of a thermosetting resin and at leastone polycarboxylic acid, and drying and heating the resultingimpregnated boards to cure said resin, said polycarboxylic acid andthermosetting resin being soluble in said volatile hydrophilic solvent,said polycarboxylic acid having a boiling point at least as high as C.,said solution having a concentration of said mixture betweenapproximately 5 and 60% and said thermosetting resin constitutingbetween approximately 5 and 50% of said mixture. ,7

8. The method of stabilizing fiber [wall] boards, consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a solution in a volatilehydrophilic solvent of a mixture of a thermosetting resin and adipicacid, and drying and heating' the resulting impregnated boards to curesaid resin, said thermosetting resin being soluble in said volatilehydrophilic solvent, said solution having a concentration of saidmixture between approximately 5 and 60% and said thermosetting resinconstituting between approximately 5 and 50% of said mixture.

9. The method of stabilizing fiber [wall] boards, consisting essentiallyof felted cellulose pulp fibers against expansion and contraction withchanges in atmospheric humidity, which process comprises, impregnatingsaid boards substantially throughout said boards with an impregnatingcomposition consisting essentially of a solution in a volatilehydrophilic solvent of a mixture of a thermosetting resin and sebacicacid, and drying and heating the resulting impregnated boards to curesaid resin, said thermosetting resin being soluble in said volatilehydrophilic solvent, said solution having a concentration of saidmixture between approximately 5 and 60% and said thermosetting resinconstituting between approximately 5 and 50% of said mixture.

10. The method of stabilizing fiber [wall] boards, consistingessentially of felted cellulose pulp fibers against expansion andcontraction with changes in atmospheric humidity, which processcomprises, impregnating said boards substantially throughout said boardswith an impregnating composition consisting essentially of a solution ina volatile hydrophilic solvent of a mixture of a thermosetting resin anda polycarboxylic aromatic acid, and drying and heating the resultingimpregnated boards to cure said resin, said thermosetting resin and saidacid being soluble in said volatile hydrophilic solvent, said solutionhaving a concentration of said mixture between approximately 5 and 60%and said thermosetting resin constituting between approximately 5 and50% of said mixture.

11. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers [and containing as animpregnant] impregnated substantially throughout with an impregmmtconsisting essentially of 13 [said boards] between approximately 1% and50% of a polycarboxylic acid, said polycarboxylic acid being soluble ina volatile hydrophilic solvent and having a boiling point at least ashigh as 150 C.

12. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout [said boards] with animpregnant consisting essentially of between approximately 1% and 50% ofa polycarboxylic aliphatic acid, said acid being soluble in a volatilehydrophilic solvent and having a boiling point at least as high as 150C.

13. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout [said boards] with animpregnant consisting essentially of between approximately 1% and 50% ofa polycarboxylic aromatic acid, said acid being.

soluble in a volatile hydrophilic solvent and having a boiling point atleast as high as 150 C.

14. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout [said boards] with animpregnant consisting essentially of between approximately 3% and 50% ofadipic acid.

15. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout said boards betweenapproximately 3% and 50% of sebacic acid.

16. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout [said boards] with animpregnant consisting essentially of between approximately 1% and 50% ofa mixture of a thermosetting resin and at least one polycarboxylic acid,said thermosetting resin and said polycarboxylic acid being soluble in avolatile hydrophilic solvent and said polycarboxylic acid having aboiling point at least as high as 150' C., the amount of saidthermosetting resin being between approximately 5 and 50% of saidmixture.

17. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout [said boards] with animpregnant consisting essentially of between approximately 1% and of amixture of a thermosetting resin and adipic acid, said thermosettingresin being soluble in a volatile hydrophilic solvent, the amount ofsaid thermosetting resin being between approximately 5 and 50% of saidmixture.

18. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout [said boards] with animpregnant consisting essentially of between approximately 1% and 50% ofa mixture of a thermosetting resin and sebacic acid, said thermosettingresin being soluble in a volatile hydrophilic solvent, the amount ofsaid thermosetting resin being between approximately 5 and 50% of saidmixture.

19. As a product of manufacture, a fiber [wall] board consistingessentially of felted cellulose pulp fibers impregnated [and containingas an impregnant] substantially throughout [said boards] with animpregnant consisting essentially of between approximately 1% and 50% ofa mixture of a thermosetting resin and mixed polycarboxylic aromaticacids, said thermosetting resin being soluble in a volatile hydrophilicsolvent, the amount of said thermosetting resin being betweenapproximately 5 and 50% of said mixture.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS Pfefier Nov. 14,

